There is a trend that mobile phones or personal computers (PCs) with radio capability have multiple purposes and multiple functions. The above trend requires an antenna device which may be operable in multiple frequency bands or in a broad frequency range.
For the above requirement, e.g., the applicant applied for and obtained a patent on an invention of a built-in antenna of a radio apparatus which is operable in multiple frequency bands having impedance that may be smoothly matched, as disclosed in Japanese Patent Publication (Toroku), No. 3775795.
Another example of related art is a microstrip antenna disclosed in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2006-157954, formed on a dielectric substrate and made operable in a broad frequency range.
The above built-in antenna of the “Toroku” reference includes a first antenna element being folded and a second antenna element being open-ended, both of which share a feeding point. The first antenna element and the second antenna element have a relatively lower resonant frequency and a relatively higher resonant frequency, respectively. The first antenna element has a forward path and a backward path short-circuited to each other at a shorting bridge which may be selectively located for impedance matching of the second antenna element.
As the resonant frequency of the second antenna element becomes higher, the shorting bridge has to be located closer to the feeding point for better impedance matching of the second antenna element. It may cause, however, the impedance to be highly inductive at the resonant frequency of the first antenna element.
As described above, there is a tendency that the impedance of each of the first antenna element and the second antenna element may not be separately adjusted if their resonant frequencies are spaced to some extent. As a separation of the resonant frequencies becomes greater, this tendency becomes clearer. Hence, it could be difficult to adapt the above built-in antenna for multiple functions of a radio apparatus supposing multiple frequencies which are spaced to some extent.
The above microstrip antenna of the “Kokai” reference includes a nearly T-shaped planar element and a linear element having a meander type portion, both being formed on a dielectric substrate and facing a ground pattern. It is mentioned in the “Kokai” reference that an electric performance of the microstrip antenna may be separately controlled in a 5 GHz band and in a 2.4 GHz band due to such an arrangement of the microstrip antenna.
The above microstrip antenna, using the dielectric substrate which is usually expensive, could hardly be applied to and built into mobile phones or PCs due to cost consideration. Apart from the cost, FIG. 4 of the above “Kokai” reference could suggest a built-in antenna formed by a flat element having a certain width, connected to a feeding point, and a branching (nearly T-shaped) linear element located at a side of the flat element opposite to the feeding point.
The above flat element having the certain width and connected to the feeding point, however, may have to be located closer to a ground circuit as the suggested antenna is built into an apparatus of a smaller size and a thinner shape. The suggested antenna may thus suffer from low impedance in such an arrangement.
A parasitic element could be added to the suggested antenna and inductively coupled to the feeding point for multiple resonances. It may be difficult, however, to locate the parasitic element close enough to the feeding point as both of them are separated by the flat element having the certain width.